In the conventional coil electric generators, the conductors are wound in such a manner that the wound coils are disposed in the axial direction, such axially disposed lengths of the coils being the effective lengths of the coils for producing the electromotive force. In such generators, the effective lengths have to be sufficiently long if a sufficient electromotive force is to be obtained, but this long effective coil length means a long rotor length. This in turn requires a high precision shaft and shaft retaining means in large electric generators.
Further, in the conventional generators, the armature coils and the field coils, especially, the armature coils are wound in a zigzag type, and the winding method for the coils are very complicated. Therefore, if the worker for the manufacturing of the generator commits mistakenly a faulty winding, a fatal defect in the generator may result.
Further in the conventional generator, the steel core of the rotor unavoidably has a large mass due to its structural nature. Therefore, it is difficult to effectively cool the rotor, and if the rotor is driven at a high temperature, the temperature of the rotor coils is also elevated, so that the insulative coatings of the rotor coils should be deteriorated before long, thereby shortening the life expectancy of the rotor coils. Further, such a high temperature of the rotor coils causes the resistance of the rotor coils to be increased, with the result that the resistance loss for the rotor coils is increased, thereby squandering energy and lowering the efficiency of the generator.
Further, the large mass of the rotor as described above also causes the eddy current loss and the hysteresis loss to be increased, ultimately lowering the generator efficiency.
Further, the large mass of the steel core of the rotor not only means a wasting of material, but also brings an undesirable effect such as handling inconvenience due to the excessive weight.
Further, in the conventional generators, the steel core of the rotor is secured with the shaft as one piece, and the shaft can not be laminated. Therefore, the eddy current loss of the shaft may be large.